Dr Abdullah said ballot boxes had been stuffed to the benefit of his rival Ashraf Ghani and he had lost trust in the electoral authorities.

"We have asked our monitors to leave their offices," he said.

A run-off vote to choose who replaces Hamid Karzai was held on Saturday. Final results are due in July.

Dr Abdullah won most votes in the first round in April, but did not secure an outright majority.

Mr Karzai, who has served two terms as Afghanistan's first and only president since the Taliban were ousted from power in 2001, is obliged by law to stand down after the latest election, which would be the country's first peaceful transfer of power.